In the manufacture of ethylene interpolymers such as ethylene interpolymerized with at least one unsaturated comonomer, a number of polymerization methods and procedures are known. For example, single site and constrained geometry catalyst systems have been disclosed for manufacturing olefin polymers with high compositional uniformity and relatively narrow molecular weight distributions.
Variations in the reactor systems used to manufacture ethylene interpolymers are also known. For example, while single site catalysis systems are disclosed to provide compositionally uniform, narrow MWD products (e.g., EXACT plastomers supplied commercially by Exxon Chemical Corporation) when employed in a high pressure polymerization system and conversely products with decreased homogeneity with respect to the short chain branching distribution and a broader molecular weight distribution (e.g., EXCEED resins supplied commercially by Exxon Chemical Corporation) when employed in a low pressure gas phase polymerization process.
While the art is replete with various products and manufacturing techniques, the known range of manufacturing capabilities still do not permit the manufacturing of ethylene interpolymer compositions characterized as having high, balanced toughness properties, good processability and improved optical properties. That is, known ethylene interpolymer compositions (either as single reactor products, multiple reactor products or polymer blends) do not exhibit the desired balance of good processability (i.e., sufficient extrusion processing characteristics to avoid, for example, excessively high extruder amperage during blown film fabrication with sufficient melt strength to permit, for example, good bubble stability to maximize output rates); balanced tear resistance; high and balanced tensile properties; high dart impact resistance; and low film haze.
The traditional polyethylene solution for achieving improved toughness properties involves manufacturing products with narrow molecular weight distributions as broad molecular weight distributions are known to yield reduced toughness properties. Beyond providing a narrow molecular weight distribution, linear polyethylenes are known to provide improved toughness properties relative to highly branched LDPE. Further, beyond merely a narrow molecular weight distribution and a linear polymer backbone, compositional uniformity has been offered for enhanced toughness properties. However, while the combination of a narrow molecular weight distribution, a linear polymer backbone and compositional uniformity may provide enhanced toughness, this combination of polymer properties invariably provides poor processability (e.g., excessively high extruder amperage).
In contrast to the combination of a narrow molecular weight distribution, increased compositional uniformity and a linear polymer backbone, to achieve the balance of good processability (i.e., resistance to melt fracture and improved melt strength) and toughness properties, Lai et al. disclose in U.S. Pat. No. 5,272,236, the disclosure of which is incorporated by reference, substantially linear ethylene polymers characterized as having narrow molecular weight distribution, high compositional uniformity and long chain branching.
Other proposed solutions for achieving balanced properties include polymer blends such as those disclosed by Kale et al. in U.S. Pat. No. 5,210,142 and Hazlitt et al. in U.S. Pat. No. 5,370,940, the disclosures of both of which are incorporated by reference. However, while such polymer blends exhibit good handling properties and processability, known polymer blends inevitably exhibit insufficient compositional uniformity to provide the desired balanced toughness properties.
Fraser et al. in U.S. Pat. No. 4,243,619 disclose a process for making film from a narrow molecular weight distribution ethylene/α-olefin copolymer composition prepared by a Ziegler catalyst system which is said to exhibit good optical and mechanical properties.
Research Disclosure No. 310163 (Anonymous) teaches blends comprising a Ziegler-Natta catalyzed ethylene copolymer and a metallocene catalyzed ethylene copolymer fabricated as cast films have improved optical, toughness, heat sealability, film blocking and unwind noise properties when compared to metallocene catalyzed ethylene polymers alone. However, the improvements in tear and ultimate tensile are not shown to be balanced.
Similarly, Research Disclosure No. 37644 (Anonymous) teaches blends of Ziegler-Natta catalyzed resins and resins made using single site catalysis system exhibit superior TD tear resistance and superior MD ultimate tensile properties. Hodgson et al. in U.S. Pat. No. 5,376,439 also describe film from a polymer blend which is said to have excellent elongation, tensile and impact properties.
WO 98/26000, the disclosure of which is incorporated herein by reference, discloses cast films prepared from interpolymer compositions comprising a substantially linear ethylene/α-olefin interpolymer and a heterogeneous interpolymer wherein the composition has an I10/I2 value of <10 and is characterized as having a log viscosity at 100 rad/s≦4.43−0.8×log(I2) or a log relaxation time>−1.2−1.3×log(I2). The reported inventive examples have an average I2 of 3.65 g/10 minutes and an average I10/I2 of 7.07 and range in Mw/Mn from about 2.14 to about 3.4 and in composition density from about 0.9118 g/cm3 to about 0.9188 g/cm3. The reported Mv1/Mv2 ratios and TREF peak temperature differentials for inventive examples range from about 0.577 to about 0.728 and from about 17 to about 24° C., respectively. However, TREF peak temperature differentials are not shown to vary with composition density and no density differential between the component polymers or component molecular weights are reported in WO 98/26000, nor is any property balance or optical improvement discussed or reported.
Hence, in spite of the above disclosures, no known ethylene interpolymer composition exhibits high, balanced toughness, good processability and good optical properties. As such, there remains a need for an improved ethylene interpolymer composition, especially for use in blown film applications. There is also a need for a process for making an improved ethylene interpolymer composition with the desired property balance. There is also a need for a process for making an improved ethylene interpolymer composition wherein the process involves polymerization using multiple reactors and the process is characterized by improved flexibility such that a broad range of product molecular weights and/or densities can be economically manufactured. There is also a need for a blown film with the desired property balance. These and other objects will become apparent from the detailed description of the present invention provided herein below.